TONY Blair flew in to the G8 summit yesterday with Anglo-Russian relations at their chilliest in decades.

His Russian counterpart, Vladimir Putin, sent shockwaves round the world with a dramatic eve-of-summit interview in which he threatened to train nuclear weapons on Europe.

In doing so, President Putin showed the Great Bear was finally preparing to stir from its long, post-Cold War hibernation.

Instantly a freeze descended in the carefully cultivated thaw which had developed between Russia and the West since the collapse of the communist Soviet Union in 1991.

Blair, already infuriated by Putin's response to the murder in London of Russian dissident Alexander Litvinenko, made it clear he would not be toyed with, saying he planned to have a "frank" exchange with the president during the summit.

Asked at Prime Minister's Question Time yesterday about growing tension, he told MPs he was "concerned about the direction Russia is heading".

He added: "It would be very sensible for the Russians to give reassurances on that. I want good relations with Russia but that can only be done on the basis that there are certain shared principles and shared values."

The two leaders came face to face at the opening banquet for the heads - and their partners - of the G8 group of industrialised nations at the start of the summit in the Baltic seaside resort of Heiligendamm.

But plans for private talks later today have been delayed until tomorrow to allow more time to discuss their differences.

The PM plans to warn Mr Putin that European businesses will not want to trade with the Great Bear unless it "conforms".

Here, we analyse the events - and personalities - that some fear may revive the Cold War...

WHY HAVE RELATIONS WITH RUSSIA SUDDENLY DETERIORATED?

THE trigger was pulled by President Putin at the start of the week when he summoned journalists from each of the G8 nations to deliver an explosive message to their governments - the Cold Thaw was off.

In a stunning series of threats, insults and warnings, Putin signalled his unhappiness with the West and, most explosively, threatened to point his nuclear weapons at Europe.

WHY WAS PUTIN SO ANGRY - AND WHAT WAS BEHIND HIS THREAT?

PUTIN is furious at American plans to site a missile defence shield in the Czech Republic and Poland - in other words, in Russia's back yard.

IS PUTIN RIGHT TO BE CONCERNED?

BOTH Tony Blair and George Bush say no. Bush insists that the technology is aimed at deflecting an attack from a rogue state such as North Korea or Iran.

Earlier this week, Bush repeated an offer inviting the Russians to have a look at the technology - and even participate in its construction.

WHY WASN'T PUTIN REASSURED BY THIS?

PUTIN says that as "rogue states" have yet to develop nuclear weapons to a stage where it would be possible to fire on the US, the shield must be aimed at Russia. He fears the system is a new version of Star Wars which will create an arms race in which cash-strapped Russia cannot afford to keep up.

There are also concerns that the US will use the shield as a means from which to spy on the former Soviet giant.

HAS ANYTHING LIKE THIS HAPPENED BEFORE?

JUST once - the Cuban Missile crisis of October 1962. The last time one of the superpowers strayed into each others' territory, the world came close to nuclear oblivion.

WOULD PUTIN REALLY CARRY OUT HIS THREAT?

IN a way it doesn't matter. Although it sounds scary, in truth it would take just a few minutes for the Russians to reprogram their systems to point at Europe anyway.

The problem is not so much the reality of what Putin is saying, but the fact he is upset enough to make such a hostile-sounding threat.

WHAT ABOUT THE LITVINENKO CASE?

IT'S a further complication. In a slap in the face to Blair, Putin used his pre-G8 summit interview to describe UK requests to extradite the main suspect in the murder of former KGB agent Alexander Litvinenko, Andrei Lugovoy, as "foolish". The PM's spokesman said: "Murder is not foolish."

In their talks, Blair will impress on Putin the importance the UK attaches to seeing justice done for the Litvinenko family.

ANY OTHER TENSIONS BETWEEN BRITAIN AND RUSSIA?

ALONG with the rest of Europe, Britain continues to be annoyed at Russia for suddenly switching off gas supplies to the Ukraine last summer.

It is felt that Putin would use Russia's gas as a weapon and Blair in particular is now keen not to rely on him for energy supplies.

This has harmed both sides - Russia, because Putin has limited his market and Europe, which now has to seek out expensive alternative sources. In the UK, to the dismay of many, this looks likely to mean more nuclear power stations.

ANYTHING ELSE?

HUMAN rights. Bush again lectured Putin over his record earlier this week in a provocative speech in the Czech Republic, which the Russians consider their zone of influence. Putin says the West employs double standards, failing to pull China up on its far worse record because of the greater economic cooperation there.

WHAT ARE THE REPERCUSSIONS OF THE ROW?

IN the short-term, Blair will be devastated if the dispute overshadows his attempts to use the summit three weeks before he stands down to agree deals on climate change and Africa, the two issues said to be closest to his heart.

Optimists say Putin may be grandstanding to appease anti-West critics back home, leaving him with space to make a deal.

COULD WE SEE A RETURN TO COLD WAR?

AT the moment it is more like a Cold Peace. But the risk is there. Critics say Bush and Blair have wasted too much energy on Iraq and not enough on the huge power that is Russia.

Blair was the first Western premier to welcome Putin to the club of world leaders. And Bush told how he "looked into Putin's soul" at their first meeting and saw a man he liked.

The fact that 16 years after the end of the Cold War they have allowed the situation to deteriorate to its current condition is bad news for all of us.